1. Field of the Invention
The present invention relates to interfaces in a communication network and more particularly to an interface subsystem for use in a data or telecommunications network employing the interchange of binary signals, the interface subsystem being adaptable to a plurality of electrical interface standards in the industry, such as EIA-232-D, and CCITT Recommendations V.35 and X.21.
II. Prior Art
Virtually all communications machines that use telecommunications links employ one of a plurality of standard electrical interfaces whose specifications have been developed by the Electronics Industries Association (EIA) in the United States and by the International Telegraph and Telephone Consultative Committee (CCITT in other countries of the world. The EIA and CCITT interfaces specify voltage levels whereby control and data signals are exchanged between two business machines, such as data terminal equipment (DTE) and a data circuit-terminating equipment (DCE), in two-level form. All data signals are sent across the interface, from a transmission medium, such as a cable, using a predetermined electrical interface having a two-level, bit-by-bit serial signaling convention.
Because the processing systems of a DTE and a DCE can not, in most cases, process the communication signals having parameters conforming to the predetermined electrical interface, the signals must be converted to a different voltage level so that they may be processed. Normally, an interface circuit is utilized by the DTE and DCE for converting the communication signals to levels usable by each business machine. Similarly, the interface circuit converts the signals to be transmitted by each business machine to levels conforming to the predetermined electrical interface.
A problem arises, however, where it is desired that a DTE or DCE be used with a number of different EIA and CCITT standards. For example, it may be desirable that a DTE or DCE be able to interface with EIA RS-422-A, EIA-232-D, and CCITT Recommendation V.35. Each of the three electrical interface standards has different defined voltage levels than each of the other electrical interface standards. For example, EIA RS-422-A specifies that the entire common mode voltage (V.sub.cm) range for a receiver in a DTE or DCE shall be +7 Volts (V) to -7V and that the receiver shall operate with a maximum differential signal of 6V applied across its terminals. In contrast, EIA-232-D specifies a range of +15V to -15V and also specifies the circuit should not fail for voltages of +30V to -30V. Furthermore, V.35 defines the input voltage for the receiver to be common mode +2V to -2V and differential +0.66V to -0.66V. As a result, interface circuits cannot accommodate each of the standards with a common receiver as one receiver cannot accommodate the complete "window" of voltage levels specified by the various electrical interface standards.
Presently, this problem is solved by DTEs and DCEs each having number of interface boards, each corresponding to a particular electrical interface standard, kept available for being switched into and out of the respective DTE or DCE depending upon which electrical interface standard is being implemented at the time. Obviously, this requires that particular boards be kept available at the DTE or DCE. These boards, while not being used, are left idle and can be easily lost or damaged.
In other DTEs and DCEs, this problem is solved by having a separate cable for each of the interface standards and by routing the signals, depending upon which standard is being used, to a corresponding receiver or set of receivers. For example, a common connector is used at the interchange interface. A group of pins on that connector is dedicated to signals which are unique to the EIA-232-D interface standard. Those signals are routed to a particular receiver or set of receivers configured to receive those signals and convert them to transistor-transistor logic (TTL) level so that a communications processor in the DTE or DCE may process them. Another group of pins are dedicated to those signals unique to the RS-422-A electrical interface standard. Similarly, those RS-422-A signals are routed to receiver(s) configured to convert those signals to TTL level. The same is true for the V.35 standard and so forth. A switch is utilized to switch the appropriate receiver outputs to the communications processor. Cable identification (ID) bits in the cable are used by the switch to identify which electrical interface is being used. This method, however, consumes too much board and connector space and requires unneeded components.